Optical shaft angle encoders are used for resolving the position or measuring rotation of a shaft. Such encoders include a light source for emitting a light beam, a code wheel for modulating the light beam in response to shaft rotation, and a photodetector assembly for receiving the modulated light and producing electrical signals indicating the amount of light received by the photodetectors.
As the light is modulated in response to shaft rotation, each electrical signal from the photodetector assembly produces a waveform. The position of the shaft determines the position of each signal on its particular waveform, i.e., the phase of each signal. Therefore, the electrical signals from the detectors can be used to indicate shaft rotation. The encoder may be used as a signal commutator, where the electrical signals are used to control field currents in a brushless D.C. motor.
Some shaft angle encoders have a code wheel with alternating opaque and transparent areas, and the light source and photodetectors are on opposite faces of the code wheel. Other shaft angle encoders have reflective areas so that the light source and photodetectors can be on the same face of the code wheel.
Optical position encoders operate using a similar principle and use a code strip rather than a code wheel. Linear motion of the code strip past a light source produces a modulated optical signal that is detected by one or more optical detectors.
As the light is modulated in response to shaft rotation, each electrical signal from the photodetector assembly produces a waveform. The electrical signal is compared to a threshold signal to produce a square wave or digital output signal. One problem that occurs is that the maximum level of light reaching an optical detector may vary, dependent upon the strength of the light source, and variations in the optical properties of the code wheel or code strip. For optimal performance, the threshold voltage level should be varied accordingly. One approach to this problem is to include a monitor track on the code wheel. The monitor track constantly detects the maximum light reaching a monitor photodetector. In one example, the area of the photodetector is selected to be half that of a primary photodetector, so that the output from the monitor photodetector can be used at the threshold level for a comparator.
A disadvantage of this approach is that the extra track on the code wheel increases the size of the code wheel and also increases the size of the associated photodetector array.